<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        body {
            height: 2000px;
        }

        #box1 {
            width: 100px;
            height: 100px;
            background-color: red;
            margin-top: 100px;
        }

        #box2 {
            width: 100px;
            height: 100px;
            background-color: blue;
            margin-top: 100px;
        }
    </style>
</head>

<body>
    <div id="box1"></div>
    <div id="box2"></div>

    <script>
        const io = new IntersectionObserver((entries) => {
            console.log(entries);
        }, {
            threshold: [0, 0.25, 0.5, 0.75, 1],
            // root: document.getElementById('container')
        })

        io.observe(document.getElementById('box1'))

    </script>
</body>

</html>